The present invention relates to a continuous X-ray generation method and apparatus in which pulsed X-rays are continuously generated from plasma produced by converging and irradiating a repetitive-shot pulsed main laser beam having high peak power onto a target. In particular, the present invention relates to a continuous X-ray generation method and apparatus in which the above-mentioned target comprises a cryogenic target layer formed by the use of a rare gas or a chemically inert cryogenic (low-temperature) material having a gaseous phase at room temperature that is liquefied or solidified by cooling and in which the above-mentioned X-rays are continuously generated at a stable high output level over a long period of time when the pulsed main laser beam is converged and irradiated onto the cryogenic target layer.
It has been known from the 1970's that high-temperature high-density laser plasma is generated by converging pulsed laser light having high peak power onto a point having a diameter of 100 .mu.m or less to irradiate a solid target therewith and that the laser plasma emits pulsed X-rays of high brightness as laser plasma X-rays.
As regards the use of the laser plasma X-rays, a medical application was disclosed or proposed in US patent specification "P. J. Mallozzi et al; U.S. Pat. No. 4,058,486 (Nov. 15, 1977)" and an article "Journal of Applied Physics Vol. 45, pp. 1891 (1974)", as early as in 1974. In 1978, D. J. Nagel and his group disclosed the possibility of application to a light source for X-ray lithography in US patent specification "D. J. Nagel et al.; U.S. Pat. No. 4,184,078 (Jan. 1980)". Afterwards, it was revealed that the spectral intensity of the X-rays has laser wavelength/intensity dependency, target element dependency, or the like.
As a target which generates the plasma in response to an incident laser beam converged thereon, a cylindrical rotary drum target and a strip target were disclosed in US patent specification "J. M. Forsyth et al; U.S. Pat. No. 4,700,371 (Oct. 13, 1987)". However, since each of these targets is made of a solid material mainly comprising a metal such as copper (Cu), aluminum (Al), or gold (Au), there arise problems that the material around the converging point is vaporized by laser heating and deposited on the inside surface of the surrounding chamber wall and on the surface of the expensive X-ray mirror for converging the laser plasma X-rays which are emitted, that fine particles are ejected to damage the surface of the X-ray mirror, and so on. In view of the above, the use is restricted to a laboratory light source operated for a short period of time. Specifically, since the solid material of the target that is deposited onto the surface of the X-ray mirror strongly absorbs the X-rays, the reflectivity of the mirror is reduced so that the effective intensity of the available X-rays is decreased with time. It is therefore necessary to periodically replace the expensive X-ray mirror.
In order to solve the above-mentioned problems, A. L. Hoffman et al proposed in "Vacuum Science and Technology B3(1), pp. 258,1985" the technique of providing a mechanical shutter 12 in the space extending to the surface of the X-ray mirror, as shown in FIG. 1 appended to the present specification. Furthermore, it was proposed by N. Kandaka et al in "Japanese J. Applied Physics 37, L174-L176, 1998" to prevent the above-mentioned vaporized gas molecules from flowing to the X-ray mirror by means of a high-density buffer gas region 24 created by a gas cell 23, as shown in FIG. 2 appended to the present specification.
Also, since the rotary drum 11 (FIG. 1) or the strip 22 (FIG. 2) used as the target is repeatedly exposed to converging irradiation by the laser light, these targets are short in life due to vaporization and wear and must frequently be replaced.
In order to overcome the above-mentioned problems, a conventional continuous X-ray generation apparatus of the type described uses a cryogenic target layer made of a cryogenic material obtained by cooling and liquefying or solidifying a chemically inert material having a gaseous phase at room temperature, for example, a rare gas.
For example, U.S. Pat. No. 4,866,517 discloses an apparatus for continuously supplying a cryogenic target made of a cryogenic material obtained by cooling and liquefying or solidifying, as shown in FIG. 3.
Specifically, as shown in FIG. 3, a vacuum chamber 31 is provided with a belt conveyor 32 arranged therein and having a rotary endless belt 33 continuously movable. The cryogenic target material, liquefied or solidified, is supplied from a cryogenic material supply path 34 onto the surface of the rotary endless belt 33 and deposited thereon to form a cryogenic target layer 35.
On the other hand, a pulsed laser beam is incident into the vacuum chamber 31 through an incidence port 36 to form a converging irradiation spot 37 on the cryogenic target layer 35 deposited on the surface of the rotary endless belt 33 which is moving. At the converging irradiation spot 37, the cryogenic material of the cryogenic target layer 35 is converted into plasma to emit pulsed X-rays. The pulsed X-rays are led out through an X-ray emission port 38.
For the cryogenic target layer 35 converted into plasma and cratered at the converging irradiation spot 37, the cryogenic material is continuously supplied through the cryogenic material supply path 34 to the surface of the rotary endless belt 33 which is moving, thereby restoring the cryogenic target layer 35.
In the above-mentioned continuous X-ray generation apparatus in which the cryogenic target layer is transferred to the converging irradiation spot of the laser beam by the use of the rotary endless belt, the following problems arise.
The first problem is that the rotary endless belt, which is cooled only when it is brought into contact with a rotary element at cryogenic temperature, is subjected to bending and extending actions at cryogenic temperature and is therefore shortened in life. Furthermore, it is difficult to maintain stable low temperature at the surface to which the supplied cryogenic material is deposited.
The second problem is as follows. In the cryogenic target layer formed by the cryogenic material supplied through the supply path and deposited on the rotary endless belt, it is difficult to achieve uniform thickness along the surface. In this event, the relative position of the converging irradiation spot of the laser beam with respect to a direction normal to the surface of the cryogenic target layer may be deviated from a predetermined position, depending on the location on the surface, and therefore the intensity of the X rays generated may be unstable.
The third problem is as follows. The cryogenic material supplied from the cryogenic material supply path is directly deposited onto the rotary endless belt at poor efficiency. This means that the cryogenic material gas having a relatively high density is present in the vicinity of the converging irradiation spot of the laser beam. Accordingly, the X-rays once generated are reabsorbed by the cryogenic material gas itself to thereby effectively decrease the efficiency of X-ray generation.
On the other hand, in order to shorten the restoration time of the craters produced by the pulsed laser beam for plasma generation, the pressure or the density of the plasma generating target gas within the vacuum chamber must be increased. However, not only due to the above-mentioned third problem but also because the incident laser light itself is absorbed by the plasma generating gas within the vacuum chamber to cause gas discharge before reaching the converging irradiation spot of the laser beam, the laser plasma can not be generated at the converging irradiation spot of the laser beam.
The fourth problem is that, in case where the speed or the kinetic energy of fine particles produced by fragmentation of the target material is high, there still remains the risk of causing unrecoverable mechanical damage to the multi-layer film at the surface of the X-ray mirror due to bombardment of the fine particles.
This is because, after generation of the laser plasma by the pulsed laser light having high peak power, fine particles or debris produced by fragmentation of the target material are generally discharged from the target material following the vaporization of neutral gas molecules having a relatively high temperature. It has been reported that the multi-layer film on the surface of the X-ray mirror is damaged even by the fine particles from the cryogenic target layer, particularly when the fine particles have a diameter not smaller than 5 .mu.m and a speed not lower than about 1 km per second.
It is therefore an object of the present invention to solve the above-mentioned problems and to provide a continuous X-ray generation apparatus which does not require the replacement of a target substrate coated with a cryogenic target layer for generating plasma in response to converging irradiation of a laser beam, which is capable of continuously generating pulsed X-rays having a stable high average output level even by irradiation of a repetitive-shot pulsed laser beam having high peak power, and which is capable of continuously generating laser plasma X-rays having a stable high average output level without causing X-ray optics including an X-ray reflecting mirror for converging the generated laser plasma X-rays to be damaged by high-speed fine particles ejected from a target material.